Typically, preinsulated pipe used, for example, in district heating pipeline, comprises an inner metal pipe, which is insulated with suitable foam, said foam coated with an outer polymer surface jacket. The preinsulated pipe is made in lengths, each length having a short area at each end for which the foam coating and outer surface jacket is absent, to allow the exposed ends of the pipe to be welded together at a pipe joint. Once the pipe is welded together at the pipe joint, one of a variety of available casings is used to cover and protect the pipe joint. For example, the casing may be in the form of a heat shrinkable casing applied around the welded pipe joint. In this case, the casing is fitted to the pipe joint, then heat shrunk down onto the edges of the polymer surface jacket proximal to the joint. The casing is longitudinally wide enough to overlap the polymer surface jacket of the two sections of pipe. The overlapping area has a suitable adhesive between the casing and the jacket to provide a seal, as described for example in U.S. Pat. No. 4,521,470, which is incorporated herein by reference.
Such casings can be pre-formed cylindrical casings, which are (in pre-shrunk state) of a slightly larger diameter than the pipe. In the case of such casing, the casing is slid around one of the pipes before the pipe joint is welded, then positioned around the pipe joint after the welding of the two pipes. Such casing may also be made from a flexible sheet or film, which is positioned around the circumference of the pipe joint after the pipe joint is welded. In this case, the flexible sheet or film typically has two opposed, overlapping edges, lying longitudinally across the pipe joint; these overlapping edges are bonded or fused together before the casing is heat shrunk.
In many cases, the casing is bonded to the polymeric outer surface jacket of the pipe using an adhesive, which is either applied to the outer surface jacket or which is pre-existing as a separate, inner layer of the casing.
Once the casing is bonded or fused to the polymeric outer surface jacket of the pipe on either side of the pipe joint, the area surrounding the pipe joint, between the casing and the pipe, is filled with suitable insulation, typically in the form of a foam which is pressure injected through a small hole in the casing. A second small hole in the casing allows the air being displaced by the foam to exit the area. In this manner, the casing is, in effect, creating a mold that surrounds the exposed pipe area around a pipe joint, which is then filled with foam insulation, preferably and typically similar in insulation characteristics and/or composition to the foam coating found under the outer surface jacket.
Once the casing and foam is applied, typically, the pipe joint has similar or better characteristics, in terms of strength, rigidity, and insulation value, as the rest of the pipe.
Thus, a known method of installing a pipeline in the field includes (1) welding together the exposed ends of a pipe at a pipe joint; (2) applying a casing in the form of a flexible sheet having a first, adhesive layer and a second, polymeric layer, so that the flexible sheet overlaps the outer surface jacket of the two pipes being connected; (3) bonding the overlapping edges of the flexible sheet to form a casing surrounding the pipe joint, so that the first, adhesive layer becomes an inner layer; (4) heat shrinking the casing around the pipe joint, while simultaneously but indirectly heating the inner adhesive layer of the casing to bond the casing to the polymeric outer surface jackets of the two pipes on either side of the pipe joint; then (5) injecting foam insulation into the gap between the pipe joint and the casing. Often, such a method also requires pre-heating of the polymeric outer surface jacket of the two pipe sections in order to help activate the adhesive and promote the bond.
A second, known method of installing a pipeline in the field includes (1) sliding a pre-formed cylindrical casing around the exposed end of a pipe, and displacing it so that the exposed end of the pipe is exposed and accessible; (2) welding together the exposed ends of a pipe at a pipe joint; (3) sliding back the casing so that it covers the pipe joint and so that the casing overlaps the outer surface jacket of the two pipe lengths being connected; (4) heat shrinking the casing around the pipe joint, while simultaneously but indirectly heating the inner adhesive layer of the casing to bond the casing to the polymeric outer surface jackets of the two pipes on either side of the pipe joint; then (5) injecting foam insulation into the gap formed between the pipe joint and the casing. Often, such a method also requires pre-heating of the polymeric outer surface jacket of the two pipe sections in order to help activate the adhesive and promote the bond.
For both of these methods, each side of the casing can be heated and bonded to the outer surface jacket sequentially by applying heat to one end of the casing, then to the other, or simultaneously by applying heat to both ends of the casing at the same time. In many cases, there is as much as a 1 inch gap between the outer surface jacket and the casing; part of the challenge in applying a casing is to provide a uniform gap, and shrinking the casing evenly around the outer surface jacket.
In these known methods, typically the heat shrinkable casing is made from a cross-linked polyethylene or an uncrosslinked polyethylene. The shrinking is affected by applying heat to the casing. The common method used in the industry to apply the heat by use of torch flame with a suitable gas fuel such as propane. In the process the casing shrinks and provides hoop stress to conform to the underlying substrate and also affects the bonding of the adhesive to the substrate. Typically the end zones of the casing overlapping onto the mainline jacket are heated and shrunk. The widths of these end zones vary from 50 mm-250 mm.
When heat shrinkable casings are applied over the pipe joint and shrunk down using a hand held flame torch, this manual operation produces an imperfect installation because of uneven heating. Artful application of the torch is critical. For example, windy conditions may spread the flame and shrink the edges of the casing prematurely. Further, unless the torch is moved carefully, the torch flame may burn the casing and cause it to split. Where a large area needs to be heated, it becomes difficult or impossible to maintain the heat while the casing is being shrunk; this leads to wrinkling of the casing, imperfect installation due to trapped air, tearing, or scorching of the heat shrink material. Sometimes, it also results in improper or incomplete adherence of the heat shrink material around joint, especially at the bottom. The district heating pipelines are usually laid in trenches with two lines running in parallel, one supplying the hot water and a return line bringing back cold water to the central station. The spacing between these pipes and also the to the adjacent walls of the ditch are often quite narrow, with typically only 12 inches-40 inches available between the pipes, and as little as 7-9 inches of clearance between the pipe and the bottom of the trench. Therefore there is little room to maneuver the torch flame to apply even and effective heating on the casing all the way around. In many cases, the application and shrinking of a casing is done in a remote, awkward location, and as such, it is highly advantageous when the heating device or apparatus, or, for example, the torch flame, is portable and can be easily carried and maneuvered by one person. In other cases, the application and shrinking of a casing is done very close to a road or walkway—often less than 25 feet away, and thus the application and shrinking must be done in a safe and efficient manner. Quite often, use of open torch is prohibited. The trench in which the pipes are located is often quite narrow, and it is not easy for a person to get down into it while carrying heavy equipment. Typically, in residential areas, the district heating pipelines share paths with other pipe and telecom networks, and as such there are many obstructions from crossing pipe networks and telecom and services cabling. The trenches are muddy, wet, and dirty, and unfortunately, typically, the cleaner and dryer the area, the better the bond between the casing and the pipe.
Before the casing is applied over the joint, the joint should be prepared in the following manner: the jacket pipe coating is cleaned and imparted roughness by abrading or light grit blasting. The joint usually requires preheating to remove moisture, but more importantly to achieve certain temperature to activate the adhesive to obtain a good bond. For example, for polyethylene type shrink casings, where a typical adhesive may have melting point of around 90° C., the preheat of the pipe is often 60-90° C. This, of course, can vary depending on application and service conditions.
The sizes and configurations of torches and heating implements vary greatly in the field, as do the sizes and configurations of the pipes to be treated. Sometimes, large powerful torches are used. These tend to flare out greatly and do not allow focused heating of the casing. Sometimes four torches are used to shrink a casing to get fast production rates, with two operators on one side of the pipe and two on the other, especially for pipes of large sizes. This practice makes it near impossible to apply even, consistent heat throughout the area to be heated, to accurately control the amount of heat applied to different areas to be shrunk, or to accurately control the order in which the various areas are to be heated. Often, in order to obtain proper adhesion of the casing, it is required to maintain a minimum preheat temperature of the substrate. When less torches are employed, certain areas, for example, the outer surface jacket adjacent the opposite end of the joint, tends to have cooled below the minimum preheat temperature, so that the casing does not bond thereto. Therefore during the shrinking operation, extra prolonged heat has to be applied to casing area overlapping onto the mainline coating in order raise the adhesive-outer surface jacket interface to sufficient temperature to achieve a sound bond. With the flame torches, this is difficult as prolonged heating can scorch and damage the casing, and sometimes lead to splitting. The need for the extra prolonged heating is exacerbated by the fact the substrate is cooling down since there is gap between the casing and the substrate jacket pipe. Therefore, focused prolonged heating is imperative to achieve a good bond on the overlap jacket. Presence of wind and inclement weather would only aggravate this problem. The prolonged extra heat can also scorch and damage the adjacent polymeric jacket and also create gassing in the underlying foam insulation.
The ability to obtain a solid, strong bond between the outer surface jacket and the casing is key for maintaining the longevity of the pipeline. Repairing or re-casing joints that have failed is very expensive and difficult, since, in many cases, the pipeline is buried. One problem with torch heating is that the casing tends to shrink into the gap between the outer surface jacket of the two pipes being connected. This creates a discrepancy in the diameter of the pipe, a smaller amount of insulation at the joint, and weakening of the casing at the joint. One way to avoid such unwanted excess shrinkage is to provide heat shields, which add expense and complexity.
The present invention provides apparatus that at least in preferred embodiments may avoid the above-noted problems. Specifically, the apparatus is compact, lightweight, scalable and modular, robust and environmentally tolerant, inherently safe and reliable, similar in operation to existing field equipment, flexible, adaptable, and simple to make, use, and operate.